1. Field of the Invention
The present invention relates to a two-fiber directional line switching ring network, more particularly a loopback switching operation in such a ring network.
One of the new synchronous digital hierarchy networks now being promoted as 8 standard in North America is the synchronous optical network (SONET) system. In a ring network of the SONET system, a ring is comprised of a pair of optical fibers. One of these two optical fibers performs the transmission of data in the clockwise direction, while the other performs the transmission of data in the counterclockwise direction. Each optical fiber is divided inside into time slots for the working line and time slots for line protection. When a line disconnection or other fault occurs, the working line time slots in one optical fiber are looped back to the line protection time slots of the other optical fiber so as to protect the line and maintain communications in the network.
2. Description of the Related Art
As will be explained later with reference to the attached drawings, loopback switching was basically performed as follows in the related art.
Assume a line disconnection occurs in a transmission route in the above ring network. When this happens, a loopback switching operation is performed by a selector in a first node (A). In the time slot interchange (TSI) in the first node (A), the transmission data is transferred (from working line slots #1 to #24 to line protection slope #25 to #48) and the transmission route is looped beck. Here, the transmission data follows the path of the second node (B).fwdarw.first node (A).fwdarw.second node (B).fwdarw.third node (C) to reach the original terminating fourth node (D). At the TSI in the fourth node (D), the transmission data is transferred (line protection slot #25 to #48 to working line slot #1 to #24) and the transmission data is dropped from the fourth node (D).
In this conventional loopback switching operation, there were at least the following four problems:
(1) The above-mentioned selector became larger in size. PA1 (2) The above-mentioned time slot interchange became larger in size. PA1 (3) The control became complicated. PA1 (4) The data switching time became longer.